Yeh the PH. Every time I have measured mine it has been close enough for me. Mostly dont do it because I ran out of solution powder to calibrate my ph meter and I never want to adjust the solution to the correct temp on brew day when I'm calibrating it. I will start to do this..eventually. I'm blessed with soft water in the pacific northwest though so it works for most lagers and all the breweries I have talked to just add some gypsum for the hoppy varieties. I do want to do a side by side of the same exact beer though. One using distilled adding minerals, and the other straight tap.